1. Field of the Invention
The present invention relates to color imagers and, more particularly, to a color imager and a low-cost method of forming the imager.
2. Description of the Related Art
A photodiode is a semiconductor device that, when exposed to electromagnetic radiation, such as light, generates electron-hole pairs at depths within the device where the electromagnetic radiation is absorbed by the device. By collecting the photo-generated electrons, a relative measure of the intensity of the electromagnetic radiation can be determined.
FIG. 1 shows a cross-sectional diagram that illustrates a prior-art photodiode 100. As shown in FIG. 1, photodiode 100 includes a lightly-doped, p-type region 110, an overlying n-type region 112, and a depletion region 114 that is formed across the pn junction of p-type region 110 and n-type region 112.
In operation, photodiode 100 is first reset by placing a reset voltage on n-type region 112 that reverse biases the pn junction. The reverse-biased voltage, which sets up an electric field across the junction, increases the width of depletion region 114 so that a predefined portion of the electromagnetic spectrum can be absorbed in depletion region 114.
Once photodiode 100 is reset, photodiode 100 is then exposed to a source of electromagnetic radiation for a period of time, known as an integration period. When photodiode 100 is struck by electromagnetic radiation during the integration period, the radiation penetrates down to an absorption depth that depends on the wavelength of the radiation.
For example, blue light has an absorption depth of approximately 0.7 microns measured down from the top surface of n-type region 112, while red light has an absorption depth of approximately 1.2 microns measured down from the top surface of n-type region 112. Green light, in turn, is absorbed at a depth that lies between blue and red light.
Photodiode 100 absorbs the different wavelengths of light at the different absorption depths and, in response, generates a number of electron-hole pairs. As a result, the radiation from the predefined portion of the electromagnetic spectrum forms a number of electron-hole pairs in depletion region 114.
The electric field set up across the reverse-biased pn junction attracts the electrons that are formed in depletion region 114 (along with the electrons that are formed in p-type region 110 within a diffusion length of depletion region 114) to n-type region 112 where each additional electron reduces the magnitude of the reset voltage that was placed on n-type region 112.
Thus, at the end of the integration period, the total number of electrons collected by n-type region 112 has reduced the reset voltage to an integrated voltage. As a result, the total number of electrons collected by n-type region 112 during the integration period, which represents the intensity of the source of the electromagnetic radiation, can be determined by subtracting the integrated voltage from the reset voltage.
A black and white imager is a device that utilizes a large number of photodiodes to generate an image that is based on the intensity information collected by each photodiode. Similarly, a color imager is a device that utilizes a large number of color photodiodes to generate an image that is based on the intensity information collected by each color photodiode.
One approach to implementing a color photodiode is to utilize a photodiode, such as photodiode 100, with a filter that lies over the photodiode. For example, a red photodiode can be formed by using a first photodiode that is covered by a red filter, while green and blue photodiodes can be formed by using second and third photodiodes, respectively, that are covered by green and blue filters, respectively.
The red filter of the first photodiode passes only red light which, in turn, allows the first photodiode to produce and collect only red photo-generated electrons. Similarly, the green filter of the second photodiode passes only green light which allows the second photodiode to produce and collect only green photo-generated electrons, and the blue filter of the third photodiode passes only blue light which allows the third photodiode to produce and collect only blue photo-generated electrons.
One drawback with this approach is that the filters are not part of a standard CMOS process flow and, as a result, substantially increase the cost of a color imager with respect to a black and white imager. As a result, there is a need for a low-cost approach to forming a color imager.